Begging the question (or petitio principii, "assuming the initial point") is a type of logical fallacy in which the proposition to be proven is assumed implicitly or explicitly in the premise. The first known definition in the West is by the Greek philosopher Aristotle around 350 BCE, in his book Prior Analytics, where he classified it as a material fallacy. Begging the question is related to the circular argument, circulus in probando (Latin, "circle in proving") or circular reasoning, though these are considered absolutely different by Aristotle.
The term was translated into English from Latin in the 16th century. The Latin version, Petitio Principii (petitio: petition, request; principii, genitive of principium: beginning, basis, premise of an argument), literally means "a request for the beginning or premise." That is, the premise depends on the truth of the very matter in question.
The Latin phrase comes from the Greek en archei aiteisthai in Aristotle's Prior Analytics II xvi:
Thomas Fowler's Deductive Logic (1887) argues that the Latin origin is more properly Petitio Quæsiti, which is literally "begging the question".
The fallacy of petitio principii, or "begging the question", is committed "when a proposition which requires proof is assumed without proof." More specifically, petitio principii refers to arguing for a conclusion that has already been assumed in the premise, in effect "begging" the listener to accept the "question" (proposition) before the labor of logic is undertaken. The fallacy may be committed in various ways.
When the fallacy of begging the question is committed in a single step, it is sometimes called a hysteron proteron, as in the statement "Opium induces sleep because it has a soporific quality". Such fallacies may not be immediately obvious in English because the English language has many synonyms; one way to beg the question is to make a statement first in concrete terms, then in abstract ones, or vice-versa. Another is to "bring forth a proposition expressed in words of Saxon origin, and give as a reason for it the very same proposition stated in words of Norman origin", as in this example: "To allow every man an unbounded freedom of speech must always be, on the whole, advantageous to the State, for it is highly conducive to the interests of the community that each individual should enjoy a liberty perfectly unlimited of expressing his sentiments."
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